Charging the batteries in itself will cause resistance which will stop the prop.
The production/generation of electricity is governed by demand. You don’t produce more electricity than you can use either as power or storage.
In the grid system there is little or no storage so the electricity is produced on demand.
Charging the batteries in itself will cause resistance which will stop the prop.
Depends what your descent speed is. On the Velis it doesn’t. There are two available prop options one of which (wide chord) is significatively better at regen charging.
T28 wrote:
In descent with the power at zero kWh the propeller windmills and charges the batteries.
Basic theory of wind turbines suggests that to be a rather undesirable method of energy conversion, unless the aim is also to reduce velocity of the aircraft. Think of it this way:
For a propeller it is the thrust you want. For a wind turbine you don’t care about the thrust, but there is no way you can produce power from the wind without it. However since a wind turbine is fixed to the ground, it doesn’t move in the direction of the thrust, and there is no power conversion in that “direction” for the turbine. Power equals thrust times velocity, and therefore zero velocity results in zero power. The thrust is there in full, but all the electric power is converted from the torque. The thrust is in other words used to reduce the velocity of the wind, and then converting it into electric power.
Now, if the wind turbine is free to move in the direction of the thrust, starting in the opposite direction for instance as is the case with an aircraft windmilling. Then, depending on the ratio of thrust and aircraft(wind turbine) mass, the power would also be converted to stop the aircraft/wind turbine:
For a very low thrust to weight ratio that will work, but you will only produce a tiny amount of power, like charging a small 12V battery or something. You could say that it’s possible to use the breaking energy to charge the batteries, but this also assumes there is some benefit in reducing the speed of the aircraft, which there very well may be for landing for instance. Instead of using flaps for a steep descend which only dissipates energy, you could instead use the prop to harvest some of the energy.
The thing is though, an aircraft is (most of them at least) extremely efficient in converting potential energy to kinetic energy. It certainly is better to use the prop and charge the batteries instead of using flaps for a steep descent, but if the aim is to use as little energy as possible, it would be much better to cruise in to land at best L/D with a feathered prop.
Apologies, I thought we were talking about the prop being stopped on the ground such as when the aircraft is at a holding point.
It’s hard to understand what’s being said when (if I understand correctly) units of energy (kW-hr) are being used to describe a situation of zero power output (i.e. 0 W).
gallois wrote:
Apologies, I thought we were talking about the prop being stopped on the ground such as when the aircraft is at a holding point.
Yes well, I find it hard to believe that anyone finds it odd that a propeller driven by an electric motor don’t need to run when not in use. Is there any examples in general of electric motors running on “idle” when not in use, instead of standing still?
My original question was simply “what happens when you pull the power lever to the zero position?”. The context, which I didn’t give, was an aeroplane on the ground – perhaps waiting at the hold. I wondered whether it stopped the prop completely, or reduced it to some speed so slow as to provide no noticeable thrust.
Graham wrote:
My original question was simply “what happens when you pull the power lever to the zero position?”. The context, which I didn’t give, was an aeroplane on the ground – perhaps waiting at the hold
Indeed, and I was wondering about the exact same thing. What does zero position even mean? On an EV the throttle is designed so that at zero lever (pedal) movement, the motor will do one of two things:
This means that zero pedal movement is a very different thing if the vehicle is standing still, or it is moving. Hence; what is zero position? Looking only at the vehicle moving, then zero position means zero torque. The pedal applies torque from some negative value to a whole lot of positive value. Zero position is perhaps 10-20% pedal movement at a position where the motor has zero torque.
Assuming Pipistrel use the simplest method, this will be to adopt the car principle to a fixed pitch prop. That is the throttle governs the torque. All that is needed is IMO to remove the first point above, and we are left with a throttle that apply torque to the prop, and with a zero point position somewhere around 10-20 % or whatever is suitable. At standstill and the prop lever at zero, the prop will stand. It will stand because there are no air flow to move it and produce negative torque. At flight speed and with the throttle above zero point, it will produce positive thrust. At zero point, it will produce neither positive or negative. Below zero point, it will charge the batteries, the prop will work as a wind turbine, slowing the aircraft down at the same time. Max negative torque is probably not much.
LeSving wrote:
Hence; what is zero position?
Maybe it would have some intermediate detent, like you pull it back nearly all the way and you get the prop going round very slowly and not doing anything useful – like an avgas engine with the throttle at idle. Then pull it back a little further through a gate to the final postion and the prop actually stops?
Perhaps pilots might find it unnerving if you pull the power right back and the prop actually stops?
In the air, with the airflow, and short of some drastic manoeuver, the prop doesn’t stop.
On the ground you have to energise the high-voltage batteries and the throttle controller (two separate actions) to allow motor operation.
As a safety measure the motor will not start if you turn the throttle controller on with the throttle lever out of the idle stop.
